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Патент USA US3034877

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3,034,870
Patented May 15, 1962
1
2
3,034,870
It is accordingly a primary object of this invention to
provide an economical and ef?cient method for drying
electrolytes suitable for use in the fusion electrolysis of
METHOD OF REMGVENG AIR AND WATER CON
TAMENANTS TRAPPED WITH Tl-E CRYSTAL
STRUCTURE OF SODIUM CHLORHDE
these metals.
Other objects are to provide a more effi
cient method for drying alkali and alkaline earth metal
halides prior to their use as components in electrolytic
bide Corporation, a corporation of New York
baths from which reactive metals are to be deposited,
No Drawing. Filed Dec. 24, 1958, §er. No. 782,617
and to provide alkali and alkaline earth metal halides
6 Claims. (Cl. 23-293)
having a sufficiently low elemental oxygen and nitrogen
This invention relates to a process for removing the last 10 content to permit the deposition of reactive metal of ex
traces of air and water from salts used as constituents of
ceptional purity from an electrolytic bath of such salts.
electrolytic baths for the production of reactive refractory
The above objects are achieved by heating the elec
metals of groups IV, V, and VI of the periodic table.
trolytic salt to a temperature between its melting point
‘One commonly employed method of producing reactive
and boiling point in an environment free of deleterious
refractory metals such as titanium, zirconium, and the
contaminants such as oxygen, nitrogen, and water and
like is by electrolysis of the metallic compound dissolved
maintaining said evironment at a pressure not exceeding
in an electrolyte consisting of fused alkali or alkaline
atmospheric pressure during the heating period to purify
earth metal halides. Electrolytes commonly utilized in
said salt and render it substantially free of said Water and
this connection are calcium chloride, mixtures of calcium
air constituents.
and magnesium chlorides, mixtures of calcium chloride, 20 In order to achieve optimum results, the salt may be
Ralph M. Starla, Ramsey, NJ, assignor to Union Car
sodium chloride, and potassium chloride, as well as so
dium and potassium chlorides, either alone or in com
bination.
preheated for a length of time at a relatively low tem
perature to remove some of the contained water prior to
the more complete removal described above. This heat
Major dif?culties in the production of reactive refrac
ing step may be accomplished either under vacuum or at
tory metals arise due to their easy contamination by 25 atmospheric pressure. A standard drying oven and a
oxygen, moisture, and nitrogen introduced into the elec
temperature of about 100° C. may be used, for example.
trolytic cells with the electrolyte, either as air voids or
One of the advantages of this invention is the fact that
entrapped with mother liquor in the crystallized salt. Of
the process is quite ?exible insofar as temperature, pres
these impurities, oxygen is perhaps the most deleterious
sure, and environment are concerned. The temperature
since its presence renders the subsequent puri?cation of
must be above that required to melt the salt and below
the metal very di?icult and, in some cases, virtually im
its boiling point. The lower the absolute pressure, the
possible.
A water content as low as 0.2 percent of the
Weight of a particular electrolytic salt, a content actually
quite low when one considers the hygroscopic nature of
some compounds found satisfactory as major constituents
of electrolytic baths (for example, calcium chloride), is
sul?cient to prevent the production of ductile, high-purity
metal in large crystal form, as is required by increasingly
more rigorous metallurgical standards. Nitrogen and
nitrogen~bearing contaminants are known to exert an em
more thorough the removal of contaminants. However,
if the surface of the salt is ?ushed with an inert gas, the
pressure may be as high as atmospheric with satisfactory
results.
The time required for the removal of contaminants
depends upon a great number of variables. For any
particular case, the point at which the purging is substan
tially complete may be determined by observing the vapor
40 pressure, in the case of vacuum heating, or by measuring
brittling effect on refractory metals.
the dew point of the e?luent gas, in the case of an inert
gas flush. For example, if a salt is heated through its
been suggested and tried. Efforts to eliminate the occlu
melting point while contained in a vacuum of approxi
sion of water and air by modi?ed recrystallization proce
mately 25 microns of mercury, absolute pressure, the re
45
dures ‘were unsuccessful. In this connection, attempts
lease of Water, air and other contaminants formerly en
Several attempts to overcome these di?iculties have
were made to produce monolithic crystals by decreasing
trapped in the crystal structure will be followed by an
solubility through the addition of acid or methanol, and
increase in pressure-to 500 microns, for example. As
by the slow evaporation of the salt solution under vac
the contaminants are removed by the vacuum pump, the
uum. In allcases the products obtained had microscopic
pressure will slowly return to a value approximating the
50 original. At that point, the puri?cation step is substan
internal chambers containing either air or brine.
It has also been proposed to dry electrolytic salts at
tially complete.
200° C. at atmospheric or reduced pressure.
If the total
Similarly, if the dew point of an e?luent inert gas is
volume of the electrolyte is quite small, the amount of
measured, a parallel phenomenon will be observed whereby
water remaining after the above treatment does not ap
the dew point will increase to a high value and will then de
55
pear detrimental, but if the bath volume is increased by
crease when removal is complete, often to values as low as
as little as a weight factor of 2, the quality of the metal
produced decreases very sharply. Completely inadequate
results are
more than
Another
which the
obtained by this method with cells containing
about 5 pounds of electrolyte.
procedure involves a pre-electrolysis stage in
cell is operated under reduced voltage for a
considerable length of time, moisture being removed by
electrolysis of the oxygen-bearing compounds, by absorp
tion by the metal deposited during that period, and by
the heat of the bath. This process requires the discard
ing of the metal ?rst produced, and is, therefore, eco
nomically impractical.
—80° F. or —90° F.
It will be understood that so many variables in?uence
the actual removal times that precise limitations cannot
be attached thereto. The particular vacuum pumping
system employed, for example, as well as deposits in the
piping, dryness of the inert gas source, and the type of
chamber employed will all have some bearing on the time.
In the following examples, times have been chosen based
65 on either dew point measurements or pressure measure
ments.
Several modi?cations of the basic invention are pos
sible. In one embodiment, the electrolyte material is main
There is a great need for an ef?cient and cheap method
tained at an absolute pressure not exceeding 300 microns
which will remove the contaminants and thereby eliminate 70 for a period of 4 to 6 hours, the temperature being held
their deleterious effects on the reactive metals produced
at 50° C. to 100° C. above the melting point of the mate
by fusion electrolysis.
rial treated.
3,034,870
3
An alternative method consists in initially heating the
material to a temperature slightly below its melting point
ll
specimen of the metal obtained using this electrolyte
was 99 Brinell.
Example III
in an atmosphere of inert gas at a pressure not greater
than 3 centimeters of mercury, and preferably not exceed
ing 100 microns; subsequently increasing the temperature
slightly over the melting point of the treated material, and
adjusting the pressure to a value not exceeding 3000
microns, preferably while ?owing through the heating
About 17 pounds of reagent grade sodium chloride
were dried for 23 hours at 200° C. at a pressure of 3
centimeters of mercury. It was then heated in an en
closed chamber to a temperature between 800° C. and
850° C. for a period of 14 hours. Approximately 8400
chamber dry, inert gas at a rate of about 3 to 5 liters
liters of argon were passed through the chamber at atmo
per minute.
10 spheric pressure during this period, and, based on dew
‘A further modi?cation of the invention is to ?rst heat
point measurements, 16 grams of water or 0.2 percent
the electrolytic material or salt under vacuum at a tem
of the original salt weight were removed. The salt thus
perature well below its melting point, and subsequently
treated was used in an electrolytic bath for the produc
to heat the material to a temperature slightly above its
of tantalum metal, which exhibited a hardness of 170
melting point at a pressure not exceeding atmospheric in 15 tion
Brinell.
the presence of an inert gas, passing such gas through a
A similar quantity of reagent grade sodium chloride
melting chamber at a flow rate of 8 to 12 liters per minute.
was also conventionally dried for 24 hours at a tempera
In accordance with this variation of the invention, the salt
may be heated even at atmospheric pressure.
The ?rst two modi?cations of the invention will re
move moisture at a faster rate than is the case with the
third procedure. Investigation has shown that both vac
uum treatments will satisfactorily remove moisture in a
period of 5 to 7 hours, and that 13 to 15 hours are required
ture of 200° C. under a pressure of 3 centimeters of mer
cury.
An electrolytic bath for the production of tan
20 talum was prepared from this salt, and the metal de
posited therefrom was found to have a hardness of 253
Brinell, a value much higher than that exhibited by the
metal produced from electrolytes treated according to
method of this invention.
for the third method, when treating about 17 pounds of 25 theAnother
important effect achieved by this purification
sodium chloride. In the three methods, the length of time
procedure,
and which is believed to be directly related
of treatment is proportional to the quantity of material
to the reduction of the impurity content of the metal was
being puri?ed, and to the ?ow rate of the inert gas.
the increase in crystal size of the deposited metal. Metal
Close control of temperature is required to reduce volati
obtained by the use of impure electrolytes is ?nely di
lization of the salt with consequent risk of condensation 30 vided and powdery, whereas the metal produced from an
within, and plugging of any associated piping. Any inert
electrolyte puri?ed according to the method of this in
monatomic gas such as helium, argon or neon may be
vention is in the form of dendritic crystals, which are
used.
usually from 1 to 2 millimeters in diameter and 4 to 5
The instant invention has been successfully applied to a
in length. This increase in crystal size is
considerable number of electrolytes, and the following 35 millimeters
desirable since large crystals are not as affected by atmos
speci?c examples are given for a clearer understanding of
pheric oxidation, and may be remelted to secure massive
the invention, and to illustrate the principles and broad
metal more easily than the ?nely divided material.
application of its methods. As is well known, the hardness
Another signi?cant advantage possessed by the larger
of reactive refractory metals is sharply increased by the
crystal form is the greater ease and rapidity with which
presence of small amounts of such impurities as oxygen, 40 spent electrolyte can be leached from the metal. Such
nitrogen, carbon, and hydrogen, so that hardness values
leaching may be quickly and simply accomplished with
are generally used as an indication of metal purity. Ac
hot water without danger of deleterious surface oxida
cordingly, hardness values will be given in these examples
tion. The need for acids or special chemicals to prevent
for later comparison with that of the metal obtained from
hydrolysis and surface oxidation as required in the case
salt treated according to prior art.
45 of powdery metal is accordingly eliminated.
Example 1
Approximately 17 pounds of reagent grade sodium
chloride were pretreated by vacuum drying for 23 hours
The operation of the cell is improved by greater salt
purity. The cell atmosphere is clear, and less salt than
usual is transported from the bath by vaporization and
entrainment in the etliuent gases. The e?iciency of cur
at 200° C. and an absolute pressure of 3 cm. mercury. It 50 rent utilization and the metal recovery are both increased
was then heated under pressure not exceeding 300 microns,
and usually maintained near 100 microns, within the tem
perature range of 850° C. to 900° C. for 6 hours. No inert
gas was passed through the treatment chamber during this
since there is no waste of energy through the electrolysis
used as an electrolyte in the production of tantalum metal
the fusion electrolysis production of reactive refractory
of oxygen-‘bearing compounds.
From the foregoing it will be apparent that the in
vention provides a new method for purifying electrolytes
treatment. The salt so treated was used as an electrolyte 55 and improves upon prior methods which have been tech
in the deposition of tantalum metal. The metal thus ob
nically and economically impractical. The electrolytes
tained was found to have a hardness of 77 Brinell.
so produced are free of the least traces of air and water,
and their use in electrolytic baths for the production of
Example II
reactive metals results in a very pure, highly ductile prod
Seventeen pounds of reagent grade sodium chloride were 60 uct having large crystal size.
vacuum dried for a period of 23 hours at 200° C., and a
As used in this speci?cation, the term “periodic table”
pressure of 3 centimeters of mercury. Following this
refers to a periodic chart of the type found at pages 56
treatment the salt was heated to 770° C. under a pressure
and 57 of Lange’s Handbook of Chemistry, eighth edi
of 25 to 50 microns, and when the temperature had be
tion, 1952, published by Handbook Publishers, Inc.
This is a continuation-in-part of my copending appli
come equalized throughout the mass, the cell pressure was 65
allowed to increase to 2000 microns by regulating the addi
cation Serial No. 427,886, ?led May 5, 1954, and en
titled “Improvement in Process for Fusion Electrolysis
tion of argon to a flow rate of 4 liters per minute. The
of Reactive Metals,” and now abandoned.
temperature was then raised to 825 to 850° C., and main
What is claimed is:
tained within this range for a period of 6 hours. The dew 70
point of the e?luent gas issuing from the treating chamber
l. A method of removing water and air constituents
entrapped within the crystalline structure of sodium
was initially 65° F., but decreased to —20° F. by the end
chloride intended for use as electrolyte components in
of the treatment. The salt so treated was collected and
by electrolysis. The hardness of a 100 gram arc-melted 75 metals of groups IV, V, and VI of the periodic table
3,034,870
6
which comprises heating said halide to a temperature
between its melting point and boiling point in an en
vironment free of deleterious contaminants, and main
halide to a temperature between its melting point and
taining said environment at a pressure not exceeding at~
mosphen'c pressure during the heating period to purify
said halide and render it substantially free of said water
and air constituents.
2. A method of removing water and air constituents
entrapped within the crystalline structure of sodium chlo
ride intended for use as electrolyte components in the 10
fusion electrolysis production of reactive refractory
metals of groups IV, V, and VI of the periodic table
which comprises heating said halide to a temperature
between its melting point and boiling point in an en
boiling point in an environment free of deleterious con
taminants, and maintaining said environment at a pres
sure not exceeding atmospheric pressure during the heat
ing period to remove the remaining water and air.
5. A method of removing Water and air constituents
entrapped within the crystalline structure of sodium chlo
ride intended for use as electrolyte components in the
fusion electrolysis production of reactive refractory
,metals of 1groups IV, V, and» VI of the periodic table
which comprises initially heating said electrolyte com
ponent to a temperature below its'melting point at a
pressure not exceeding 3 centimeters of mercury, in
creasing the temperature above said melting point, de
vironment free of deleterious contaminants, maintaining 15 creasing the pressure to a value not exceeding 3000 mi
said environment at a pressure not exceeding atmospheric
crons, passing a dry, inert gas over said electrolyte com
pressure during the heating period, and ?owing a dry, in
ponent to purify said electrolyte component and render
ert gas across the surface of said halide to purify said
- it substantially free of Water and air constituents.
halide and render it substantially free of said water and
6. A method of removing water and air constituents
20 entrapped within the crystalline structure of sodium chlo
air constituents.
3. A method of removing water and air constituents
ride intended for use as electrolyte components in the
entrapped within the crystalline structure of sodium chlo
fusion electrolysis production of reactive refractory met
ride intended for use as electrolyte components in the fu
als of groups IV, V, and VI of the periodic table which
sion electrolysis production of reactive refractory metals
comprises placing said electrolyte ‘component in a heat
of groups IV, V, and VI of the periodic table whichicom 25 ing chamber, reducing the pressure to a value not ex
prises heating said halide to a temperature between its
ceeding 300 microns, heating said electrolyte component
melting point and boiling point in an environment free
at a temperature 50° to 100° C. above its melting point
until measurements indicate substantially no e?lux of
of deleterious contaminants, maintaining said environ
ment at a pressure not'exceeding atmospheric pressure
moisture, and cooling said electrolyte component where
during the heating period, and ?owing a dry, inert gas 30 by the puri?ed electrolyte component is obtained in the
selected from the group consisting of helium, argon, and
solid state.
neon across the surface of said halide to purify said
halide and render it substantially free of said water and
air constituents.
4. A method of removing water and air contaminants 35
entrapped within the crystalline structure of sodium ‘chlo
ride intended for use as electrolyte components in the
fusion electrolysis production of reactive refractory
metals of groups IV, V, and VI of the periodic table
which comprises initially heating said halide to a tem 40
"t"a».at
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,564,498
Nisbet ______________ __ Aug. 14, 1951
2,762,684
Wainer ______________ __ Sept. 11, 1956
OTHER REFERENCES
Mellor’s “Modern Inorganic Chemistry," 1939, Long
perature below its melting point and at a pressure not ex
ceeding atmospheric pressure to remove substantial
mans, Green and Co., N.Y., pp. 160-161.
Kroll et al.: “Bureau of Mines Report of Investiga
amounts of water and air therefrom, further heating said
tion #4915.”
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